Several hardening methods are described in the literature that are implemented in static environments. In particular, there is plasma nitriding by means of a low temperature plasma gas intensified by a thermionic emission source (U.S. Pat. Nos. 5,294,264 and 5,443,663), a bath of salts (U.S. Pat. Nos. 5,518,605; 6,645,566), powder (U.S. Pat. No. 6,105,374), and by means of low temperature ion nitriding (U.S. Pat. No. 6,179,933). A technique of ion implantation has been proposed (U.S. Pat. Nos. 5,383,980; 6,602,353).
There also exists a non-static method in which a laser beam that is movable relative to the substrate is directed onto the substrate and produces surface melting in the impact zone. Nitrogen is blown onto the substrate in a direction that remains fixed relative to the direction of the laser beam, and an inert gas is also blown onto the piece (EP-A-0 491 075). In that method, the nitrogen is mixed with the inert gas and both the laser beam and the nitrogen-inert gas jet converge on the piece so that the gaseous mixture strikes the liquid zone. To prevent said zone being converted into a spray, it is necessary to limit the pressure of the gas jet. This method has made it possible to obtain hardening of a Ti alloy over a thickness of 400-1000 microns.
U.S. Pat. No. 3,944,443 describes the application of a high temperature induction plasma with a combination of nitrogen gas with either propane or BF3 to achieve hard surface layers up to 250 microns. The object to be coated must be electrically isolated.
U.S. Pat. No. 4,244,751 describes melting the surface (but does not describe ionizing the nitrogen molecules) of Al with a plasma torch (TIG) to obtain a hard surface. The thickness of the surface layer is <200 microns.
U.S. Pat. Nos. 5,366,345 and 4,451,302 describe hardening of a metal substrate using a laser or e-beam with melting of the surface in nitrogen.